This invention relates generally to heat shrinkable, relatively gas and moisture impermeable, thermoplastic packaging films which can be heat sealed to itself to form a flexible package. The invention relates more particularly to packaging films for food products such as bakery goods, snack foods, cheese products, and candy where a high moisture vapor barrier and oxygen barrier packaging material is required.
There is a need in the food packaging industry for a packaging film which has improved moisture vapor and oxygen barrier characteristics. A packaging film for use in connection with food products should exhibit several desirable characteristics. The film should be heat shrinkable so as to provide an attractively packaged food product. Additionally, the film should be heat sealable and have seal integrity under processing conditions, and in particular the heat sealed seams should resist being pulled apart during heat shrinking operations. The film should also be heat sealable to itself. Furthermore, the food contact layer of such a film must qualify under the appropriate food laws and regulations for safe food contact. Very importantly, the film must provide a good oxygen and vapor barrier, i.e. must possess a low permeability to both oxygen and water vapor to maintain the freshness of the food contained therein. Also, the film must possess sufficient shrink energy such that upon the heat shrinking of a food product in a package made from the film in accordance with the invention, the film will shrink snuggly around the product contained therein, representatively about 30 to 50% biaxial shrinkage at about 90.degree. C. In addition to the foregoing, the film should desirably possess optical clarity i.e. the film should not become cloudy upon exposure to conditions of heat shrink, so that the consumer appeal of the packaged product is maintained.
In general, such a multi-layer film structure will have the minimal structure (sealing and food contact layer/shrink layer/barrier layer/sealing and food contact layer), a composite structure being required to achieve the desired composite properties of the packaging film. A preferred barrier material is unplasticized saran which refers to a family of thermoplastic resins, such as vinylidene chloride polymers, and copolymers of vinylidene chloride with other monomers such as vinyl chloride, acrylonitrile, methyl methacrylate, acrylates, acrylate esthers, and vinyl acetate. Copolymers of vinylidene chloride with other monomers are generally described in terms of the weight ratio of the monomer units in the copolymer. Saran by definition, has at least 50% vinylidene chloride. The unplasticized saran of the present invention is a low viscosity saran.
A heat shrinkable, thermoplastic, multi-layer packaging film which has enjoyed considerable commercial success for making bags having low gas transmission is described in U.S. Pat. No. 3,741,253 issued on June 26, 1973 to Brax et al, which relates to a multi-layer film comprising a first outside layer of an irradiated ethylene vinyl acetate copolymer, a core layer of polyvinylidene chloride copolymer, and a second outside layer of an ethylene vinyl acetate copolymer. Typically the process of manufacturing this type of oriented heat shrinkable film is a tubular orientation process wherein a primary tube of the film is biaxially oriented by stretching with internal pressure in the transverse direction and the use of pinch rolls at different speeds in the machine direction. This "blown bubble" technique is well known in the art. After the bubble is collapsed, the film is wound up as flattened, seamless, tubular film to be used later to make bags, for example either end seal bags typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, or side-sealed bags in which the transverse seals from the bag sides and one edge of the tubing form the bag bottom.
This type of bag is used by placing the food product in the bag, evacuating the bag, gathering and applying a metal clip around the gathered mouth of the bag to form a hermetic seal, and then immersing the bag in a hot water bath at approximately the same temperature at which the film was stretch-oriented, typically about 160.degree. to 205.degree. F., hot water immersion being one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly. One problem which has been encountered is the failure of the bag seals at the bottom of the bags as the bag is shrunk around a product, the shrink forces tending to pull the seal apart.
Of interest concerning the present invention, is the disclosure of U.S. Pat. No. 3,808,304 for "Oriented Blends of Polypropylene and Poly-Butene-1" issued Apr. 30, 1974 to Schirmer, being directed to packaging film made of such blends and having improved heat sealing and heat shrinking properties in near boiling water.
Of interest is the disclosure of U.S. Pat. No. 3,634,553 for "Heat Shrinkable Films of Polypropylene and an Ethylene/Butene Copolymer" issued Jan. 11, 1972 to Foglia et al, being directed to oriented thermoplastic films formed from blends of a high isotactic content polypropylene with an ethylene/butene-1 copolymer containing a minor amount of ethylene.
Of interest is the disclosure of U.S. Pat. No. 4,207,363 for "Flexible Heat Shrinkable Multilayer Film For Packaging Primal Meat" issued June 10, 1980 to Lustig et al, being directed to a heat shrinkable multi-layer film including a first outer layer comprising a blend of a propylene-ethylene copolymer, a butene-1/ethylene copolymer, and a thermoplastic elastomer; a first core layer capable of being stretched during orientation of the multi-layer film; a second core layer serving as an oxygen barrier and being compatible to the biaxial orienting and heat shrinking of the film; and a second outer layer comprising an ethylene/vinyl acetate copolymer; the multi-layer film being biaxially oriented.
Of interest is the disclosure of U.S. Pat. No. 3,891,008 for "Multilayer Thermoplastic Film and Method for Making Same" issued June 24, 1975 to D'Entremont, being directed to a polymeric composition comprising a blend of polybutene-1 with an ethylene-propylene copolymer which, when combined with a shrinkable, cross-linked polymer of ethylene produces a laminate having improved abuse and tear resistance and reduced self-adherence.
Of interest is the disclosure of U.S. Pat. No. 3,754,063 for "Method for Casting an Inflated Tubular Polyethylene Film with Mixed Polymer of Polypropylene-Polybutene-1" issued Aug. 21, 1973 to Schirmer, being directed to a thermoplastic film having improved abuse characteristics including a layer of ethylene vinyl acetate copolymer on a tubular substrate of a predominantly ethylene polymer material with an outer layer of admixed isotactic polypropylene, polybutene-1, and atactic polypropylene, the resulting film being oriented.
Of interest is the disclosure of U.S. Pat. No. 4,448,792 for "Pasteurizable and Cook-in Shrink Bag Constructed of a Multilayer Film" issued May 15, 1984 to Schirmer and assigned to a common assignee with the present application. This disclosure is directed to a multilayer film including a layer of propylene homopolymer or copolymer, a layer of a blend of the propylene polymer with butylene homopolymer or copolymer, an adhesive layer comprising a copolymer of ethylene, a fourth layer of vinylidene chloride copolymer, a fifth adhesive layer of ethylene copolymer, and another layer of propylene homopolymer or copolymer, the film being oriented and irradiated to at least a dosage sufficient to render the film delamination resistant during pasteurizing or cooking.